The present invention relates to a reader which reads reflected light or transmitted light from an original document by using a solid image sensor so as to convert its read signal into a digital image signal.
Conventionally, such a reader of this kind as shown in, for example, FIG. 1 is known. In the known reader, an original document 23 is placed face down on a glass plate 21 having a white portion 28 formed at one distal end thereof. While the glass plate 21 is being displaced by a pulse motor 10, light emitted from a white fluorescent lamp 3 and reflected at the white portion 28 is reflected by a mirror 26 and is condensed by a lens 27 so as to be received by a linear CCD (charge coupled device) sensor 4. In the CCD sensor 4a, quantity of light is converted into an electric signal and the electric signal to be outputted is adjusted to a predetermined level. Subsequently, reflected light from the original document 23 is likewise subjected to photoelectric conversion by the CCD sensor 4 sequentially such that an analog image signal is obtained.
As shown in FIG. 2, the CCD sensor 4 includes a sensor portion 31 having n CCDs Sl to Sn on the assumption that character n denotes a natural number. Photoelectric charge corresponding to quantity of light is stored in each of the CCDs Sl to Sn. The stored photoelectric charge is transferred, through a transfer gate 32 receiving a transfer pulse .phi.t, to an analog shift register unit 33 having n shift registers SRl to SRn and then, is sequentially shifted to an output buffer 34 by shift clocks .phi.1 and .phi.2. In the output buffer 34, photoelectric conversion is performed such that a voltage VO corresponding to the stored photoelectric charge and acting as an analog image signal is outputted. Meanwhile, each time photoelectric charge is shifted to the output buffer 34, the output buffer 34 resets the photoelectric charge by a resetting pulse .phi.r. Although not specifically shown, after wave form and level of this analog image signal have been adjusted by a sample holding circuit and a clamping circuit, the analog image signal is converted by an A-D converter into a digital image signal indicative of gradations of 8 bits having 0 to 255 levels.
Quantity of reflected light from a white portion of the original document to be read changes variously, according to kinds of the original documents, relative to a reference quantity of reflected light from the white portion 28 acting as a reference for adjusting while level. For example, in case the original document is a photograph having low white level, the quantity of reflected light from the white portion of the original document is about 60% of the reference quantity of reflected light from the white portion 28. On the other hand, in case the original document is a printed document having high white level, the quantity of reflected light from the white portion of the original document amounts to about 1.2 times the reference quantity of reflected light from the white portion 28.
The known reader referred to above, the quantity of reflected light from the white portion 28 of the glass plate 21 is fixed as the reference quantity of reflected light and the photoelectric conversion voltage VO corresponding to the reference quantity of reflected light is converted by the A-D converter into the digital image signal indicative of full range, i.e. 255 gradations. Therefore, in case the original document is the photograph, the maximum gradation, i.e. the white level does not exceed the 160th gradation and thus, a range of high gradations is not utilized at all. Accordingly, such drawbacks are incurred that dynamic range of image data becomes narrow and S/N ratio deteriorates. On the other hand, the white level of the digital image data read from the printed document reaches as high as the 300th gradation, so that accurate image data cannot be obtained since a portion of high gradations is converted into the 255th gradation.